Combined antiretroviral therapy has increased dramatically the life span of HIV-1 infected individuals, but damaging effects of HIV-1 persist throughout the nervous system, and the prevalence of HIV-1 associated neurocognitive disorder (HAND) is greater than 50% of HIV-1 infected people in the USA. Our long-term goal is to understand the pathogenesis of HAND and develop therapeutic interventions. The proposed studies are focused to determine the novel endolysosome-dependent mechanisms whereby HIV-1 transactivator of transcription protein (HIV-1 Tat) contributes to the development of HAND. Our central hypothesis is that HIV-1 Tat induces synaptic disruption and neuronal injury by elevating endolysosome pH and activating a novel endolysosome-dependent calcium regulatory mechanism. Guided by our preliminary findings, this novel hypothesis will be tested by pursuing three specific aims. (1) Determine, in vitro, the extent to which HIV-1 Tat activates a novel endolysosome-dependent calcium regulatory mechanism. (2) Determine, in vitro, the extent to which the above endolysosome-dependent calcium regulatory mechanism underlies HIV-1 Tat-induced neuronal injury. (3) Determine, in vivo, the extent to which the above endolysosome mechanism contributes to disrupted synaptic integrity in HIV-1 Tat transgenic mice. Our results are expected to demonstrate that endolysosomes play a critical role in HIV-1 Tat-induced synaptic disruption and neuronal cell death through a novel endolysosome-dependent calcium regulatory mechanism that is upstream of previously described effects of HIV-1 Tat. Such results are expected to link together unified mechanisms causing early pathological features of HAND. Results from this work will lead to a greater understanding of HAND pathogenesis and the possible development of new therapeutics.